1. Field of the Invention
The present invention relates a magnetic head and, particularly, relates to a magnetic head for use in video applications such as a video tape recorder (VTR), 8 mmVTR (for example, see NIKKEI ELECTRONICS 1983.5.23, pages 111-124), an electronic still camera (magnetic disc video recording and reproducing apparatus: for example, IEEE TRANSACTIONS ELECTRONICS Vol CE-28, No. 3, August 1982, "The Electronic Still Camera A New Concept In Photography").
2. Description of the Prior Art
A magnetic recording apparatus for recording and reproducing information through the use of a magnetic medium has been playing an increasing important role in the information processing oriented society. Such apparatus has been widely used in many technical fields such as audio, video and digital machines. Of these, since the advent of the video tape recorder for recording and reproducing video for broadcasting, it has been remarkably developed and has been commercially available as a so-called home video as well as a broadcasting video. In addition, recently, the 8 mm video tape recorder and the electronic still camera have been developed. A major basis for such remarkable development largely depends on advancement of high density magnetic recording technique using a magnetic head and tape.
As far as a magnetic tape for high density magnetic recording is concerned, there is a tendency that material used for such magnetic tape progressively changes from originally used magnetic materials of oxide, such as .gamma.-Fe.sub.2 O.sub.3, Co..gamma.-Fe.sub.2 O.sub.3, to magnetic materials of metal, such as Fe fine particles and Co - Ni evaporation film, which have substantially strong magnetic characteristic. Following such tendency, there is also a tendency that material for such magnetic head changes from magnetic material of oxide including the presently used Mn - Zn ferrite single crystal to magnetic materials of metal, such as sendust, amorphous ribbon and Co - Zr - Nb thin film.
The magnetic head playing an important role for the above described high density magnetic recording comprises, in general, a main core formed corresponding to a track width and a pair of reinforcing cores for reinforcing the main core by sandwiching the main core between them. The main core comprises a pair of core halves which are joined to each other with a gap length. In order to record information on a magnetic tape with high density, it is necessary to make a recording frequency higher, in other words, to make the wavelength smaller, in which case a minimum wavelength (.lambda..sub.min) is determined by a magnitude of the gap length (G). The minimum wavelength is approximately equal to twice the gap length (.lambda..sub.min .apprxeq.2G). For example, assuming that the minimum wavelength is 0.5 .mu.m, the gap length of the magnetic head must be made to be 0.25 .mu.m. Accordingly, very precise machining technique is required and improvement of material suitable for magnetic recording is also required. On the other hand, recording on the magnetic tape is made in response to a leakage flux leaking to the magnetic tape through the gap length, out of magnetic flux formed between a pair of core halves. If and when a wavelength is short, a curve of leakage flux becomes abrupt, which is suitable for high density magnetic recording. Accordingly, it is an important factor for high density magnetic recording how such leakage flux is made abrupt. In other words, how a pair of core halves constituting a main core are adhered to each other is a significant problem.
Conventionally, a pair of core halves made of sendust are adhered to each other by using silver solder material (Ag - Cu alloy). However, since the silver solder partially diffuses into the core halves, joining force is increased whereas permiability of the sendust is decreased, and hence a curve of a leakage flux is broadened, which does not meet a requirement for an abrupt leakage flux curve. Therefore, another approach of using glass material has been proposed instead of a silver solder material. Unlike silver solder material, glass material is little diffused and the permiability of sendust is not adversely affected by glass. Therefore, a leakage flux curve becomes abrupt and hence, it is theoretically clear that performance can be enhanced from a view point of high density recording. Of course, permiability in the vicinity of the core gap largely affects a reproducing output even in the course of reproduction. However, since the glass comprises an oxide whereas the sendust comprises metal material, joining ability between the glass and sendust is not so good. Accordingly, there has been a dilemma between use of silver solder material and use of glass material, as an adhesive for core halves made of sendust alloy.
Now, referring to FIGS. 1 to 8, a magnetic head of the prior art will be described in the following, in a more particular manner.
FIG. 1 is a perspective view showing a main core for use in a magnetic head, the magnetic head comprising the main core made of alloy material and a pair of reinforcing cores for holding the main core so that the main core is sandwiched between the reinforcing cores, and FIG. 2 is an exploded perspective view of a conventional magnetic head. Such magnetic head is disclosed in the Japanese Patent Laying-Open Gazette No. 222427/1983, for example. Such a magnetic head comprises a main core 1 and a pair of reinforcing cores 5 for holding the main core 1 by sandwiching the main core 1 between the pair of reinforcing cores 5, as shown in FIG. 2. The main core 1 comprises, as shown in FIG. 1, a pair of core halves 1a, 1b made of magnetic material of alloy such as sendust, which are joined to each other by adhering means 2, 3 such as silver solder material. A gap length 4 is formed between the core halves through such joining, in the front of the main core 1. On the other hand, as shown in FIG. 2, the reinforcing cores 5, 5 comprised of a portion 6 of magnetic material such as ferrite and portion 7 of non-magnetic material such as soda glass are joined to the main core 1, from both sides thereof, respectively, so that a magnetic head is formed.
The basic structure of magnetic head was described in the foregoing. A conventional method for forming a main core for use in such magnetic head comprises, as shown in FIG. 3, (1) forming, in a contacting surface of one of a pair of sendust wafers 11 and 12 grooves 13 and 14 where glass sticks 16 are inserted and melted and a groove 15 where coils are wrapped later, by means of machining process, (2) providing a mirror grinding on contacting surfaces or gap-length-forming surfaces of these wafers 11 and 12 and thereafter, (3) contacting both wafers and inserting the glass sticks 16 into the grooves 13, 14 and 15 and then heating and melting the glass sticks 16 with a load F being applied. As a result, as shown in FIGS. 4 and 5, the melted glass permeates through the contacting surfaces of the wafers 11 and 12, so that a core block can be formed. Incidentally, before contacting the above described wafers 11 and 12, a film 17 of SiO.sub.2 having a thickness of 0.2 .mu.m for forming a gap length is partially formed, as a spacer, on a margin of a surface of one of the wafers 11 and 12, so that a core block with a gap length can be formed. When a final main core is completed, the portion of the film 17 of SiO.sub.2 as a spacer is removed. As described previously, since a glass generally comprises an oxide whereas alloy material comprises metal, joining ability between both materials is not so good and hence the melted glass does not fully permeate through the gapped portion as formed in advance. Therefore, the wafers 11 and 12 are partially adhered to each other. For this reason, mechanical strength thereof becomes weak and hence, if and when the core block is sliced into main core chips, as shown by lines 19 in FIG. 6, so that a main core chip as shown in FIG. 7 can be obtained, the glass 16 partially interposed in the gap, which is now solidified, is broken as shown in FIG. 8, so that the gap length 18 expands from 0.2 .mu.m to approximately 5 m. In addition, since, while glass 16 is being melted, the melted glass 16 does not fully permeate through gap and hence only a small cavity 10 is produced, there is also defect that it takes much time to form a hole for windings in the groove 5. Furthermore, there is a fear that in case of magnetic head for video applications, having extremely small track width, a curve or separation of core halves may be caused in forming a predetermined gap length (track width) by means of grinding, if and when a joining force between core halves is so small. Furthermore, the magnetic head formed by the above described conventional method is less reliable, like degradation of reproducing output due to change of gap length, since the joining force between two core halves is so small.
On the other hand, in order to make a final magnetic head stronger, it is also important to increase a joining force between a main core and a pair of reinforcing cores. Conventionally, an organic resin or non-organic adhesive is mainly used for adhering the main core to the pair of reinforcing cores so that a magnetic core is structured in a sandwiched manner. However, since the working temperature of the organic resin or non-organic adhesive is near normal temperature, such joining method has several defects in several respects. Thus, from the point of view of increasing reliability of joined portions, joining by glass material has been used. However, as described in the foregoing, the glass material comprises an oxide, whereas alloy material for main core comprises metal material, the joining ability between these is not so good, resulting in poor joining force and incomplete contact.